The present invention relates to a process for obtaining isobutene from a C.sub.4 -hydrocarbon mixture containing isobutene, by reacting the mixture with a primary C.sub.3 - or C.sub.4 -alcohol, isolating the tertiary ether formed and decomposing it at an elevated temperature.
It is already known to obtain isobutene from a C.sub.4 -hydrocarbon mixture by means of a sulfuric acid extraction process. In this process, highly concentrated sulfuric acid must be used and consequently expensive materials must be employed for the equipment. Since, furthermore, side-reactions of isobutene, for example dimerization, polymerization, hydration and the like, occur during the extraction, the sulfuric acid extraction process is not satisfactory in respect of yield, and of quality of the products.
A process for obtaining isobutene is also known, for example from German Pat. No. 1,216,865 or German Published Applications DAS No. 1,934,422 and DAS No. 2,011,826, in which a C.sub.4 -hydrocarbon mixture containing isobutene is reacted with methanol in a first stage and the resulting methyl tert-butyl ether is decomposed into methanol and isobutene in a second stage. However, the known processes have the disadvantage that methanol forms azeotropic mixtures with the C.sub.4 -hydrocarbons. For example it is known from German Laid-Open Application DOS No. 2,629,769 and German Published Application DAS No. 1,934,422 that in the preparation of methyl tert-butyl ether, when the unconverted hydrocarbons are removed from the reaction mixture by distillation they contain about 2% of methanol, due to the hydrocarbon/methanol azeotropes, and this methanol can only be recovered by expensive methods, for example by interpolating a water wash. It is a particular disadvantage that on separating by distillation the reaction mixture obtained from the decomposition stage and containing isobutene and methanol, the methanol and isobutene form an azeotropic mixture so that an expensive water wash must also be interpolated into the decomposition stage (cf., for example, German Published Application DAS No. 1,934,422) in order to minimize the loss of methanol and obtain a methanol-free isobutene, as is required for most applications.
It is true that in addition to the use of methanol and possibly ethanol as alcohols for the etherification reaction, primary alcohols in general have previously been referred to as possible reactants for the conversion to the tertiary ether (cf., for example, German Pat. No. 1,216,865 and German Published Applications DAS Nos. 1,934,422 and 2,011,826, already referred above). However, there was a substantial prejudice against the use of higher primary alcohols, for example C.sub.3 - or C.sub.4 -alcohols, since it was known that such higher primary alcohols can easily be dehydrated to undesired olefins under the reaction conditions of the decomposition stage, in the presence of an acid catalyst. For example, German Published Application DAS No. 1,934,422, already referred to, expressly points out, in column 3, 1st paragraph, that methanol, which cannot be dehydrated, should be used as the alcohol in order to avoid the undesirable formation of olefins in the decomposition stage.
A further substantial prejudice against the use of higher primary alcohols resulted from the fact that it was known, for example from U.S. Pat. No. 3,170,000, especially Table I and column 3, lines 29 to 31 that methanol and ethanol give substantially higher yields in the etherification reaction than do the higher primary alcohols, e.g., the C.sub.3 - or C.sub.4 -alcohols.
Because of the disadvantages and prejudices described above, the conventional processes for obtaining isobutene by decomposing the tertiary ether obtained in a first etherification stage have not found industrial use but have only remained prior art on paper, and hence the industrial production of isobutene had to depend on the use of the sulfuric acid extraction process, with all the shortcomings and disadvantages inherent in the said process.
In is an object of the present invention to provide a process for obtaining isobutene from a C.sub.4 -hydrocarbon mixture containing isobutene, which does not suffer from the disadvantages of the conventional processes.
We have found that this object is achieved by a simple process for obtaining isobutene from a C.sub.4 -hydrocarbon mixture containing isobutene, by reacting the mixture with a primary alcohol in the presence of an acid condensing agent and decomposing the resulting tertiary ether in the presence of an acid catalyst at an elevated temperature, wherein the primary alcohol used is a primary C.sub.3 - or C.sub.4 -alcohol and the acid condensing agent used for the formation of the ether is an ion exchanger in its acid form, and the primary C.sub.3 - or C.sub.4 -alcohol and the C.sub.4 -hydrocarbon mixture are fed, with or without prior mixing, first to the etherification reaction zone, which contains the ion exchanger, the reaction mixture obtained from the etherification reaction zone is then distilled in a first distillation zone, in which the top product, taken off without interpolating a water wash, is a C.sub.4 -hydrocarbon mixture comprising the unconverted hydrocarbons and containing not more than 1,000 ppm by weight of primary C.sub.3 - or C.sub.4 -alcohol, and the bottom product taken off is the resulting C.sub.3 - or C.sub.4 -alkyl tert-butyl ether, which may contain primary C.sub.3 - or C.sub.4 -alcohol which may have been added in excess, the bottom product is then fed to a second reaction zone, containing an acid catalyst, in which the C.sub.3 - or C.sub.4 -alkyl tert-butyl ether is decomposed at an elevated temperature to give isobutene and primary C.sub.3 - or C.sub.4 -alcohol, the mixture of isobutene and primary C.sub.3 - or C.sub.4 -alcohol is fed to a second distillation zone in which isobutene containing not more than 500 ppm by weight of primary C.sub.3 - or C.sub.4 -alcohol is taken off as the top product without interpolating a water wash and the primary C.sub.3 - or C.sub.4 -alcohol is taken off as the bottom product, and the resulting primary C.sub.3 - or C.sub.4 -alcohol is recycled to the etherification reaction zone.
Using the novel process, a C.sub.4 -hydrocarbon raffinate which is virtually free from C.sub.3 - or C.sub.4 -alcohol is isolated from the reaction mixture obtained after the etherification stage, by simple distillation without interpolating a water wash, since unconverted primary C.sub.3 - or C.sub.4 -alcohol surprisingly does not form an azeotrope with the C.sub.4 -hydrocarbons. In general, a C.sub.4 -hydrocarbon raffinate containing not more than 1,000 ppm by weight of C.sub.3 - or C.sub.4 -alcohol, preferably at most 500 ppm by weight, in particular at most 100 ppm by weight, is taken off as the top product of the distillation. Again, when the reaction mixture, obtained on decomposing the C.sub.3 - or C.sub.4 -alkyl tert-butyl ether, is separated by distillation into isobutene and the C.sub.3 - or C.sub.4 -alcohol, azeotropes of the alcohol are not formed. The C.sub.3 - or C.sub.4 -alcohol can therefore be recovered, without interpolation of a water wash, in a simple manner and virtually without losses, and be recycled to the etherification stage.
Surprisingly, the process according to the invention gives isobutene in high yield, for example in a yield of more than 97%, based on the isobutene contained in the C.sub.4 -hydrocarbon mixture employed. This was unexpected since U.S. Pat. No. 3,170,000, already referred to, states in column 3 that on using C.sub.3 - or C.sub.4 -alcohols only very poor yields of tertiary ether are obtained. It is also known from U.S. Pat. No. 3,634,535, especially column 6, that the reaction of isobutene with propanol gives the tertiary ether in a yield of only about 50%, whilst the corresponding reaction of isobutene and methanol gives yields of from about 90 to 95%. It is therefore surprising that yields of tertiary ether of more than 95% are obtained by the process according to the invention.